Are you doing them on a pull-up bar or on rings? The
Crossfit exercises page has several good videos on muscle-ups and
beastskills has a decent tutorial. I didn't have rings for a while, so I learned to do them on a pull-up bar. I got my first one at a body weight of ~195lbs (I'm 5'11") and had one elbow go over before the other (which is perfectly ok when learning bar muscle-ups). I'm still improving my form to even this out, but unless I'm quite fatigued, I can do a pretty even muscle-up now on the pull-up bar. I've only tried it on rings a few times and found them to be easier, but I guess once you have enough strength for one, the other won't be too hard.
As I was learning them, I thought for a long time that the "change over" between the pull-up and dip was the issue, but really, I just wasn't getting nearly high enough on my pull-ups. Here are a few things that helped me build explosiveness in my pull-up:
1. Kipping pull-ups (lots of good vides on the
CF exercises page)
2. Clapping pull-ups (as in
this video)
3. Weighted pull-ups
The muscle-up also involves a very deep dip, which you can improve with:
1. Weighted dips
2. Ring dips
3. Bar dips - if you are doing muscle-ups on a bar, you should definitely practice these, as they feel quite different than "normal" dips on a dip station (with parallel bars). An easy way to practice them is to lower the bar on a smith machine to ~chest high, and just jump on it with your hands under you holding the bar. Dip down as low as you can possibly go and you'll notice that you need to bring your feet out in front of you (under the bar, as if doing an l-sit) as a counterbalance. On a bar muscle-up, you start the dip with the bar around your nipples, so when practicing bar dips, dip until your chest is touching the bar.
Finally, to practice the change-over between the pull-up and dip without actually having to do a muscle-up, try "jumping muscle-ups". That is, setup a bench or something to stand on under the pull-up bar or rings. Start with a big jump that gets you well over the bar/rings and catch yourself in a support position. Once this feels easy and you can balance yourself easily, use less of a jump next time so you are forced to do a dip to get into the support position. After that, progressively use less and less of jump so you are forced to start practicing the change over. Basically, the jumping removes the (challenging) requirement of a very hard pull-up and allows you to work on the change-over and dip portion exclusively, which will make you much more likely to do them properly when your pull-up gets strong enough.